Area consideration in shock tube
 

Hello,

I'm trying to understand the behaviour of a pressure wave-front as it enters a diverging / converging section in a tube. I tried looking it up in John D. Anderson's 'Modern Compressible Flow with a Historical Approach' text, but it only discusses unsteady waves in uniform closed pipe.

OpenFoam's rhoCentralFoam has a Sod's shock-tube tutorial which deals with uniform cross-section again. I, after reading about behaviour of instruments, know when a pressure wave enters an area of greater cross-section, an expansion wave gets sent in the opposite direction. I am unable to visualize this in a converging-diverging geometry I developed using rhoCentralFoam. I'm unable to understand where I am wrong or how this solver actually considers the area term.

I require help regarding understanding the significance of area in a shock tube. Can someone please help?

Could you attach simple picture of your case ?

Hello,

Please find the attachments.

A simple introduction:
pipe_caseStraight simulates the evacuation of a high pressure chamber with a pipe connected to atmosphere.

pipe_caseStep simulates the same thing, but the pipe has a larger cross-section step in between.

Theoretically, when the pressure wave-front enters this step, a negative pressure wave should be sent towards the high pressure chamber which should aid evacuation sooner.

Refer: image 2.jpg which shows plots of pressures at a probed location. The sudden drop in pressure due to the step is not seen in the second plot. This is my confusion. Please help!

PS: Request you to run blockMesh, setFields and rhoCentralFoam. I have deleted the timestep data to reduce size.

Could you give a picture of the initial pressure distribution and the point where the pressure measurement was performed? Unfortunately I can not read the data in OpenFOAM format.

PS: Build the Mach number distribution behind the shock wave, keep in mind that weak perturbation can not propagate up the supersonic flow